1. Purpose of the Invention
This invention relates in general to certain new and useful improvements in the measurement of body tissue movement by non-invasive techniques and, more particularly, to the detection of cardiovascular abnormality and apparatus and method for monitoring the condition of the human cardiovascular system.
2. Brief Description of the Prior Art
As is well known, coronary artery disease is the leading cause of death in the United States. About half of those affected are in apparent good health and are totally unaware that they have the disease. Accordingly, doctors and researchers in this area are still striving for a simple and safe method for early detection of the disease.
The only technique widely used in screening patients for coronary artery disease is electrocardiography, which provides a graphic tracing of the bioelectric potential of the heart muscle. When electrocardiography is used for coronary screening, an individual's electrocardiogram (ECG) is recorded first while the individual is at rest, and then after strenuous exercise, on the theory that the exercise produces a transient imbalance between the heart's oxygen supply, derived from the coronary artery blood flow, and its oxygen requirements, imposed by the work of exercise. Such an imbalance may be detected by the development of specific electrocardiographic abnormalities typical of coronary artery disease. Unfortunately, however, the electrocardiogram is relatively insensitive to this oxygen imbalance, and the probability of a successful diagnosis using electrocardiography is only about sixty percent in patients with symptoms suggestive of coronary artery disease. For those individuals exhibiting no apparent symptoms of the disease, the probability of correctly diagnosing its presence by electrocardiography alone is far lower. About half of those individuals who exhibit abnormal ECG's during a stress test, but are otherwise apparently healthy, do not actually have coronary artery disease.
There have been some attempts to use apex cardiograph determination which is based on a measurement of the relative movement of small segments of the chest wall over the cardiac apex. However, this measurement is a surface phenomena, is restricted to a small area, and is also relatively impractical in that it does not represent the true motion of the heart.
At present, then, there is no acceptable method for diagnosing or evaluating the severity of coronary artery disease short of cardiac catheterization, which involves direct injection of a chemical into both the coronary arteries and the heart chambers themselves. Although this is a highly reliable diagnostic tool, it is also relatively expensive and time consuming. Moreover, it involves significant risks to the patient, and is obviously unsuited for screening purposes.
In recent years, a number of animal laboratory investigations have shown that reduction in blood supply to the heart muscle, as occurs in human coronary artery disease, can be detected by studying the movements of segments of the heart. When a segment of the heart receives less oxygen than is normally required for proper function, as occurs with a partial obstruction of a coronary artery by atherosclerosis, the normal contraction pattern of cardiac muscle changes in a characteristic fashion. In simple terms, while the normal muscle segments of the heart continue to contract inward, the afflicted segment bulges outward. Then, as normal muscle segments relax and lengthen, the previously stretched, abnormal muscle segment shortens. Thus, the abnormal segment exhibits a movement similar in contour, but opposite in direction, to that of the normal segments.
Prior to this invention no one had succeeded in developing a noninvasive technique for diagnosing coronary artery disease in humans by identifying an abnormality, of the kind just described, in the movement of heart muscle segments. It is apparent that such a technique would include the principal steps of recording movements of a selected segment of the heart, precisely timing and identifying the direction of this movement in relation to the expected, normal pattern of movement during the entire cardiac cycle of contraction and relaxation, and transiently provoking a contraction abnormality of a muscle segment by some type of stress which induces an oxygen imbalance as described previously. The closest previous approximation to this approach can probably be found in those devices which recorded movement of the chest wall induced by total cardiac movement, such as the instruments known as the vibrocardiograph, the kinetocardiograph, and the apexcardiograph. These devices have been used during exercise, but have been found to be of no value in diagnosing coronary heart disease in man, and are no longer used in this manner.
It has been recognized for some time that an instrument known as a displacement cardiograph (DCG) could be used to detect physical movement of body tissue, including muscle segments of the heart. Such an instrument was described in a German "Offenlegungsschrift", No. 1,566,044, published on Apr. 2, 1970, and a corresponding Israeli Pat. No. 26,039, dated June 29, 1966. This latter device has also been described in various other publications, for example, "Electronic Device for Physiological Kinintic Measurements and Detection of Extraneous Bodies" by Ran Vas, published in IEEE Transactions on Biomedical Engineering, Vol. BME-14, No. 1, Jan., 1967, pages 2-6, and "The Displacement Cardiograph" by R. Vas et al, published in IEEE Transactions on Biomedical Engineering, Vol. BME-23, No. 1, Jan., 1976, pages 49-54.
The last mentioned publication also describes in general outline how a displacement cardiograph might be used as an apex cardiograph in the detection of ischemia. Significantly, however, the DCG has not been utilized in the detection and diagnosis of coronary artery disease. It is also noteworthy that use of the DCG for any study of heart movements requires an accompanying recording from a conventional instrument, such as an electrocardiograph and phonocardiograph in order to relate the DCG tracing to the timing of the cardiac cycle.
The basic apparatus of the DCG includes a constant or reference oscillator circuit; a variable oscillator circuit; an inductive detector coil connected to the variable oscillator circuit so as to determine, in part, the frequency of the variable oscillator circuit; a mixer circuit connected to receive input signals from the reference and variable oscillator circuits, and to provide an output signal having a frequency equal to the difference in input frequencies; a frequency-to-voltage converter circuit; and a recording system. The detector coil, although satisfactory for some applications of the DCG, does not have the necessary sensitivity for the accurate detection of movements of heart muscle segments, and also introduces significant distortion into the signal. The other circuit components together form a frequency-modulation system for deriving a body tissue displacement signal, and such a system, although satisfactory for many applications, does not provide the high degree of sensitivity needed in the accurate monitoring of heart segment movements.
Considering the DCG apparatus more specifically, it has been observed that one of the primary disadvantages of this apparatus and the method used thereby is that the apparatus operates by measurement of an electromagnetic field at the body tissue and utilizes an inductive change. In the frequency modulation system employed by the DCG apparatus, one frequency was compared to another. However, with spontaneous changes in oscillator or cardiac frequency, the sensitivity of the detector system would be altered considerably.
The present invention is far more effective than the DCG apparatus in that place changes are measured, as opposed to frequency changes, and the phase changes are considerably larger, thereby providing a much more accurate measurement. In addition, the apparatus of the present invention is far superior to the DCG apparatus in that the signal-to-noise ratio is improved due to the stability of the oscillator forming part of the present invention, and the fact that less electronic components are used.
One of the other problems with the DCG apparatus is that it effectively, though undesirably, functioned as a metal detector. This created a serious disadvantage in that many patients have been provided with metal components, as for example, artificial heart valves, cardiac pacemarkers, pacing electrodes, titanium vascular staples, wire sutures to close the sternum, and the like. Accordingly, these metal components materially interfere with any measurement to be obtained with the DCG apparatus.
Accordingly, the DCG apparatus has not been found to be entirely suitable for use in the detection of coronary artery disease. There is, therefore, still a great need for an instrument which obviates the aforedescribed shortcomings of the DCG, and thereby provides a convenient and reliable noninvasive technique for screening individuals to determine whether or not they have coronary artery disease or other forms of atherosclerotic vascular disease. The present invention satisfies this need.